This proposal describes a series of experiments which will address how neural activity shapes patterns of synaptic connections in the developing nervous system. Once established, synaptic connections undergo a prolonged editing process which results in some functional connections becoming permanently deleted while others are maintained. Work over the last 20 years in different parts of the nervous system has shown that this synapse elimination involves competitive interactions among different inputs innervating the same target cell and is modulated by neural activity. Following this period, ongoing maintenance of synaptic connections and their use-dependent modulation continue to occur over a lifetime. How axons form synapses within a target, begin to function and become edited to give functionally relevant patterns of connectivity are poorly understood. In large part, this is because dynamic events such as these are difficult to study in vivo and have proved to be difficult to recapitulate in vitro. Neuromuscular junctions are well suited to studying activity dependent modulation of synapse formation and elimination in vivo, because they are simply organized, easily observed and manipulated, and there exists a wealth of information about their structure, function and molecular composition. We will use in vivo video microscopy and physiological analysis to study early structural and functional events during synapse formation and elimination at mouse neuromuscular junctions. We will then alter the molecular environment between motor neurons and muscle fibers in a spatially and temporally selective manner using viral mediated gene transfer. We will study the role of neural activity in modulating synaptic competition in in vitro preparations of spinal cord, nerve and muscle and with in vivo pharmacological manipulations. of particular interest is determining whether the disappearance of electrical coupling among motor neurons innervating the same muscle fiber triggers the onset of synaptic competition. These three sets of experiments are aimed at elucidating the activity-dependent and -independent mechanisms that result in functional patterns of synaptic connections throughout the developing nervous system.